Update appveyor.yml with gradlew coordinates

[aalmiray]

Let's print out the current gradle version too, just to be sure we have the right environment for a build

[mojavelinux]

Good thinking!

On a separate thread, if you want to try to add caching for the AsciidoctorJ build, feel free to add what you determine works for the Gradle plugin build. We could always use a schedule to nuke the cache on a regular basis if we are concerned about false positives.